The invention is directed to an improved technique for coding an audio signal and, in particular, to embedding a code into an audio signal so that a decoder can detect the code reliably despite signal degradation.
Audio signals are generated in a variety of ways, such as by radio and television stations, and transmitted in various ways, such as by means of airwaves, cable and satellite as well as distributed on magnetic tape and storage disc (e.g. optic, magnetic) media. Various benefits are derived from identifying these audio signals which constitute "audio products" in the form of programs or commercials, for example. The audio products can be broadcast by radio, television or cable stations and/or stored on tape, CD-ROM or other media for replay by the consumer. By being able to automatically distinguish one audio product from another, it becomes possible to perform a variety of services. For example, air time verification is possible to verify for an advertiser that a commercial has actually been broadcast, and that it was aired in its entirety, at the proper time and in the locations that were paid for. In addition, performance royalty revenues can be more accurately calculated based on the frequency with which a piece of music, say, has been broadcast. For these and other reasons, it is highly desirable to know when a particular audio product has been "performed" in the sense that it has been heard by any member(s) of a listening audience. Furthermore, the listening (or watching) audience can be measured by having individual members or individual households equipped with devices capable of identifying certain designated audio products, and then processing the resultant data. This can help measure the popularity of a program so that its value to advertisers can be assessed. Also, the exposure of an audience to a commercial can be measured this way, and such information can be combined with other data to determine the effectiveness of that commercial in terms of how well it is remembered and/or the resulting purchases made thereafter.
The automation of this identification by the prior art has involved various techniques for embedding a code in the audio product. The resulting signal is reproduced by, say, a speaker of a radio or television set. The embedded code is also reproduced by the speaker so that it can be detected by a sensing device for data storage and/or processing to yield the desired information. Various types of encoding schemes are known. However, they have proved to be unsatisfactory for one or more of the following reasons. If the code is easily removable without permission, then the accuracy of the desired measurement will obviously be skewed. Therefore, it is important for the embedded code to be "indelible" in the sense that it cannot be removed without seriously (or at least noticeably) damaging the audio product. Also, the code must not create any audible deterioration in the quality of the audio product itself, i.e. which can be discerned by a human listener. Furthermore, the code must have adequate immunity to noise which occurs during the sending, playback and receiving operations of the encoded audio product. For example, an audio product is typically exposed to various phase shifts and time shifts in the process of being recorded and/or broadcast. In addition, the audio product may be compressed by a bit rate reduction system based on psychoacoustic compression techniques, such as EUREKA 147, DOLBY AC3, and MPEG2. (The term "psychoacoustic" has to do with the human auditory response to a sound stimulus.) The code must withstand such processing while still maintaining its characteristics for enabling it to be reliably recovered by the decoder while remaining inaudible. Meeting all of these requirements has proven to be too tall of an order for the prior art, particularly when combined with the need to minimize the complexity of the apparatus and method, and to carry out the technique quickly and efficiently.